Calibration of photoelectromagnetic sensor in a laser source

ABSTRACT

In a laser-produced plasma (LPP) extreme ultraviolet (EUV) system, laser pulses are used to produce EUV light. To determine the energy of individual laser pulses, a photoelectromagnetic (PEM) detector is calibrated to a power meter using a calibration coefficient. When measuring a unitary laser beam comprising pulses of a single wavelength, the calibration coefficient is calculated based on a burst of the pulses. A combined laser beam has main pulses of a first wavelength alternating with pre-pulses pulses of a second wavelength. To calculate the energy of the main pulses in the combined laser beam, the calibration coefficient calculated for a unitary laser beam of the main pulses is used. To calculate the energy of the pre-pulses in the combined laser beam, a new calibration coefficient is calculated. When the calculated energy values drift beyond a pre-defined threshold, the calibration coefficients are recalculated.

BACKGROUND

1. Field

The present application relates generally to laser systems and, morespecifically, to calibration of a photoelectromagnetic sensor in a lasersource of a laser produced plasma (LPP) extreme ultraviolet (EUV)system.

2. Related Art

The semiconductor industry continues to develop lithographictechnologies which are able to print ever-smaller integrated circuitdimensions. Extreme ultraviolet (“EUV”) light (also sometimes referredto as soft x-rays) is generally defined to be electromagnetic radiationhaving wavelengths of between 10 and 102 nm. EUV lithography isgenerally considered to include EUV light at wavelengths in the range of10-14 nm, and is used to produce extremely small features (e.g., sub-32nm features) in substrates such as silicon wafers. These systems must behighly reliable and provide cost-effective throughput and reasonableprocess latitude.

Methods to generate EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has one or moreelements (e.g., xenon, lithium, tin, indium, antimony, tellurium,aluminum, etc.) with one or more emission line(s) in the EUV range. Inone such method, often termed laser-produced plasma (“LPP”), therequired plasma can be generated by irradiating a target material, suchas a droplet, stream or cluster of material having the desiredline-emitting element, with a laser beam at an irradiation site withinan LPP EUV source plasma chamber.

FIG. 1 illustrates some of the components of a prior art LPP EUV system100. A laser source 101, such as a CO₂ laser, produces a laser beam 102that passes through a beam delivery system 103 and through focusingoptics 104 (comprising a lens and a steering mirror). Focusing optics104 have a primary focus point 105 at an irradiation site within an LPPEUV source plasma chamber 110. A droplet generator 106 produces droplets107 of an appropriate target material that, when hit by laser beam 102at the primary focus point 105, generate a plasma which irradiates EUVlight. An elliptical mirror (“collector”) 108 focuses the EUV light fromthe plasma at a focal spot 109 (also known as an intermediate focusposition) for delivering the generated EUV light to, e.g., a lithographyscanner system (not shown). Focal spot 109 will typically be within ascanner (not shown) containing wafers that are to be exposed to the EUVlight. In some embodiments, there may be multiple laser sources 101,with beams that all converge on focusing optics 104. One type of LPP EUVlight source may use a CO₂ laser and a zinc selenide (ZnSe) lens with ananti-reflective coating and a clear aperture of about 6 to 8 inches.

The laser source 101 can be operated in a burst mode where a number oflight pulses are generated in a burst with some amount of time betweenbursts. The laser source 101 may comprise a number of lasers thatgenerate pulsed laser beams having distinct properties, such aswavelength, and/or pulse length. Within the laser source 101, the beamdelivery system 103, and the focusing optics 104, the separate laserbeams may be combined, split, or otherwise manipulated.

Before the laser beam 102 reaches the LPP EUV source plasma chamber 110,the beam 102 is measured at various points within the laser source 101,the beam delivery system 103, and/or the focusing optics 104. Themeasurements are taken using a variety of instruments that measure oneor more aspects of the laser beam 102. In some instances, the laser beam102 may be measured before it is combined with other generated beams orafter it has been combined. The instruments, however, may not directlymeasure certain properties of the laser beam 102 or may be not becalibrated in such a way as to measure the properties of the laser beam102.

SUMMARY

According to an embodiment, a system comprises: an energy monitor withina laser source of a laser-produced plasma (LPP) extreme ultraviolet(EUV) system, the energy monitor configured to measure laser pulseshaving a same wavelength and occurring in a burst, the energy monitorcomprising: a power meter configured to measure an average power of thelaser pulses over a defined period of time, and a photoelectromagentic(PEM) detector configured to provide a first voltage signal indicativeof a temporal profile of the burst of the laser pulses over at least aportion of the defined period of time; a calibration module configuredto determine a calibration coefficient based on the average power andthe first voltage signal, the calibration coefficient being a ratio ofan energy of the burst of the laser pulses determined from the averagepower and an integral of the first voltage signal; and a single pulseenergy calculation (SPEC) module configured to determine an energy of asubsequent pulse of the series of the laser pulses based on thecalibration coefficient and a pulse integral of a second voltage signalprovided by the PEM detector indicating a temporal profile of thesubsequent pulse.

The system may further comprise a recalibration module configured tocalculate an energy of a second burst based on a third voltage signalindicative of a second temporal profile of the second burst and tocompare the energy of the second burst to a second average power sensedby the power meter and to instruct to the calibration module to updatethe calibration coefficient based on the comparison.

According to an embodiment, a method comprises: measuring laser pulseshaving a same wavelength and occurring in a burst using an energymonitor within a laser source of a laser-produced plasma (LPP) extremeultraviolet (EUV) system, the measuring comprising: receiving, from apower meter, an average power of the laser pulses measured over adefined period of time, and receiving, from a photoelectromagnetic (PEM)detector, a first voltage signal indicative of a temporal profile of theburst of the laser pulses sensed during at least a portion of thedefined period of time; determining a calibration coefficient based onthe average power and the first voltage signal, the calibrationcoefficient being a ratio of an energy of the burst of the laser pulsesdetermined from the average power and an integral of the first voltagesignal; and determining an energy of a subsequent pulse of the series ofthe laser pulses based on the calibration coefficient and an integral ofa second voltage signal provided by the PEM detector indicating atemporal profile of the subsequent pulse.

According to an embodiment, a non-transitory computer readable mediumhas instructions embodied thereon, the instructions executable by one ormore processors to perform operations comprising: measuring laser pulseshaving a same wavelength and occurring in a burst using an energymonitor within a laser source of a laser-produced plasma (LPP) extremeultraviolet (EUV) system, the measuring comprising: receiving, from apower meter, an average power of the laser pulses measured over adefined period of time, and receiving, from a photoelectromagnetic (PEM)detector, a first voltage signal indicative of a temporal profile of theburst of the laser pulses sensed during at least a portion of thedefined period of time; determining a calibration coefficient based onthe average power and the first voltage signal, the calibrationcoefficient being a ratio of an energy of the burst of the laser pulsesdetermined from the average power and an integral of the first voltagesignal; and determining an energy of a subsequent pulse of the series ofthe laser pulses based on the calibration coefficient and an integral ofa second voltage signal provided by the PEM detector indicating atemporal profile of the subsequent pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a portion of an LPP EUV system according to theprior art.

FIG. 2 is a diagram of an energy monitor according to an exampleembodiment.

FIG. 3 is an illustration of a burst mode of a laser source, accordingto an example embodiment.

FIG. 4 is a graph output by a PEM detector depicting a temporal profileof a burst comprising main pulses.

FIG. 5 is a graph output by a PEM detector depicting a temporal profileof a single main pulse.

FIG. 6 is a graph output by a PEM detector depicting a temporal profileof a burst comprising pre-pulses.

FIG. 7 is a graph output by a PEM detector depicting a temporal profileof a single pre-pulse.

FIG. 8 is an example of a graph output by a PEM detector depicting atemporal profile of a pre-pulse and a main pulse separated by a lengthof time.

FIG. 9 is a block diagram of a system for measuring an energy of apulse, according to an example embodiment.

FIG. 10 is a flowchart of an example method of measuring an energy of apulse, according to an example embodiment.

FIG. 11 is a flowchart of an example method of calibrating aphotoelectromagnetic (PEM) detector using a power meter for a unitarylaser beam.

FIG. 12 is a flowchart of an example method of calibrating a PEMdetector using a power meter for a combined laser beam.

DETAILED DESCRIPTION

Within an LPP EUV system, the energy of a laser pulse is calculated atvarious locations in the laser source, the beam delivery system, and/orthe focusing optics. The sensors used in an LPP EUV system to measure alaser beam do not directly measure the energy of a pulse of the laserbeam. The sensors include a power meter that provides a measurement ofthe average power of the pulses generated over a defined period of time.The sensors further include a photoelectromagnetic (PEM) detector thatoutputs a voltage signal based on detected infrared (IR) light over alimited period of time. The voltage signal provides a temporal profileof the individual laser pulses. Using the data collected by the sensors,a calibration coefficient is calculated to calibrate the PEM detector tothe power meter. After the calibration, the energy of the pulses can becalculated from the voltage signal provided by the PEM detector.

The laser beam being measured can comprise pulses of light of the samewavelength, referred to as a unitary laser beam. The unitary laser beammay comprise either pre-pulses of a first wavelength or main pulses of asecond wavelength. To determine the energy of a pulse of light in aunitary laser beam, the PEM detector is calibrated to the power meter bycalculating a calibration coefficient for the unitary laser beam. For aunitary laser beam, the calibration coefficient is a ratio based on ameasurement received from the power meter and the voltage signalprovided by the PEM detector over a burst. After calibration, thecalibration coefficient and the voltage signal provided by the PEMdetector are used to calculate the energy of the pulses in the unitarylaser beam.

In the LPP EUV system, when two unitary laser beams of differentwavelengths are combined, the resulting combined laser beam has pulsesof alternating wavelength separated in the time domain. In theembodiments described herein, the pulses in the combined laser beamalternate between the pre-pulses of the first wavelength and the mainpulses of the second wavelength. When calculating the energy of a mainpulse in the combined laser beam, the calibration coefficient calculatedfrom the unitary laser beam of main pulses is used. Due to the differenteffects of the optical components in the LPP EUV system between thepre-pulses and the main pulse in the combined laser beam, a separatecalibration coefficient of the pre-pulses in the combined laser beam iscalculated. The calibration coefficient of the pre-pulses is determinedbased on a difference between the power measured by the power meter andthe power attributable to the main pulses in the combined laser beam.After calibration, the calibration coefficient and the voltage signalprovided by the PEM detector are used to calculate the respectiveenergies of pre-pulses and main pulses in the combined laser beam.

FIG. 2 is a diagram of an energy monitor 200 according to an exampleembodiment, comprising a power meter 202 and a PEM detector 208. Theenergy monitor 200 may receive the laser beam 102 from another componentwithin the laser source 101 using, for example, a beam splitter. Aswould be apparent to one of ordinary skill in the art, before reachingthe energy monitor 200, the laser beam 102 travels through one or moreoptical components to pick-off a portion of the laser beam 102 formeasurement. These optical components may include a diamond window, apartial reflector, or a Zinc Selenide window. An example of a laser seedmodule that can include the energy monitor 200 is discussed incommonly-assigned U.S. Patent Application Publication No. 2013/0321926published Dec. 5, 2013.

The energy monitor 200 is located so as to measure the laser beam 102 ata particular place in the laser source 101, the beam delivery system103, or the focusing optics 104. In some embodiments described herein,the placement of the energy monitor 200 causes the energy monitor 200 tomeasure a unitary laser beam 102 comprising laser pulses of light of thesame wavelength (e.g., pre-pulses or main pulses). The laser pulses oflight are generated by a single source but can be generated by more thanone source in other systems. In other embodiments described herein, theplacement of the energy monitor 200 causes the energy monitor to measurea combined laser beam 102 generated by two laser sources with differentwavelengths. The laser source 101, beam delivery system 103, and thefocusing optics 104 can include more than one energy monitor 200.

The laser beam 102 follows an optical path through the energy monitor200. The laser beam 102 is split by a beam splitter 204 so that a firstportion of the laser beam 102 continues along the optical path while theremainder of the laser beam 102 is directed to a reflector 206. Thereflector 206, in turn, directs the remainder of the laser beam 102 tothe power meter 202.

The power meter 202 is configured to measure the average power of thelaser beam 102 over a defined period of time. The measurement may span anumber of bursts of the laser beam 102. In some instances, themeasurement may span 5, 10, or 20 bursts of the laser beam 102. Thedefined period of time may be a fraction of a second or a number ofseconds. In some instances, the defined period of time is one second.

The portion of the laser beam 102 not directed to the power meter 202 isdirected to a further beam splitter 204. From the beam splitter 204, afirst portion of the laser beam is directed to, for example, furthersensors or other optical components (not shown). The remainder of thelaser beam 102 is directed to the PEM detector 208.

The PEM detector 208 provides a voltage signal that indicates a temporalprofile of the laser beam 102. The temporal profile spans at least aportion of the defined period of time used by the power meter 202. Thetemporal profile may span at least a burst of the laser beam 102. Tocalculate the energy of pulses in a combined laser beam, the temporalprofile spans a pre-pulse and a main pulse.

While only one PEM detector 208 is shown in FIG. 2, additional PEMdetectors (not shown) may be included in the energy monitor 200.Further, the laser beam 102 may be modified before measurement by thePEM detector 208 using, for example, a lens (not shown) or diffuser set(not shown). The energy monitor 200 may be enclosed by a casing andattached to a port of the laser source 101 or be enclosed within thelaser source 101.

FIG. 9 is a block diagram of a system 900 for measuring an energy of apulse, according to an example embodiment. The system 900 comprises anenergy monitor 902, a calibration module 904, a single pulse energycalculation (SPEC) module 906, and an optional recalibration module 908.The system 900 may be implemented in a variety of ways known to thoseskilled in the art including, but not limited to, as a computing devicehaving a processor with access to a memory capable of storing executableinstructions. The computing device may include one or more input andoutput components, including components for communicating with othercomputing devices via a network or other form of communication. Thesystem 900 comprises one or more modules embodied in computing logic orexecutable code.

The energy monitor 902 is configured to receive data about the laserpulses of the laser beam 102. The energy monitor 902 comprises, or is inelectronic communication with, a power meter and a PEM detector. In someinstances, the energy monitor 902 is the energy monitor 200 comprisingthe power meter 202 and the PEM detector 208. The power meter isconfigured to measure an average power of the laser pulses over adefined period of time. The defined period of time may be, for example,one second. The PEM detector is configured to provide a voltage signalindicative of a temporal profile of the laser pulses over at least aportion of the defined period of time.

When the energy monitor 200 or 902 is placed in the LPP EUV system 100so as to receive a unitary laser beam 102, the calibration module 904 isconfigured to determine a calibration coefficient based on the datacollected by the power meter (e.g., power meter 202) and the PEMdetector (e.g., PEM detector 208). A calibration coefficient iscalculated for each unitary laser beam. The calibration coefficient isused to calculate the energy of a single pulse (e.g., single main pulse502 or single pre-pulse 702) based on later-collected PEM detector data.The calibration coefficient is a ratio determined from the average powerand an integral of the voltage signal.

To determine the calibration coefficient in a unitary laser beam 102,bursts of pulses are analyzed. FIG. 3 is an illustration 300 of twobursts 302 of laser pulses, according to an example embodiment. Theillustration 300 is an outline of a temporal profile provided by the PEMdetector 208 depicted as a varying voltage as a function of time(measured in milliseconds (ms)).

Each burst 302 is depicted in the illustration 300 as a curve having arising edge 310, peaking 312, then maintaining a voltage level 314 lowerthan a peak level for a length of time before ending with a falling edge316. The burst 302 has a burst length 304 beginning at the rising edge310 and ending at the falling edge 316. When calculating the energy ofthe burst 302 to calculate a calibration coefficient as is explainedelsewhere herein, the PEM detector 208 has a scope window 306 that atleast encompasses the burst length 304. The scope window 306 can belengthened to capture the time between bursts 302 or shortened tocapture only one or two pulses within the burst 302.

A burst period 308 is measured from the rising edge 310 of a first burst302 to the rising edge 310 of a second burst 302. The burst period 308can be determined from a repetition rate or “rep rate” of the bursts 302indicating a number of bursts over a defined period of time (e.g., onesecond). The burst rep rate may be expressed as a frequency such as 5Hertz (Hz), 10 Hz, or 20 Hz.

FIG. 4 is a graph 400 depicting a temporal profile provided by theoutput of PEM detector 208 of a burst 402 of a unitary laser beam 102that comprises main pulses. The graph 400 is an actual example of theoutput illustrated in FIG. 3. The unitary laser beam 102 is generated bya single source. In an embodiment, the main pulses have a wavelength of10.59 microns. As depicted, the burst 402 lasts approximately 3.5milliseconds and comprises a pre-determined number of laser pulses.According to various embodiments, a burst 402 may comprise differentnumber of laser pulses based on the burst length. The laser pulses havea width (measured as a length of time) and an amplitude. As part ofcalculating the calibration coefficient, over a length of time shown inthe graph 400, the calibration module 904 can integrate the pulses ofthe burst 402.

FIG. 6 is a graph 600 output by the PEM detector 208 depicting atemporal profile of a burst 602 of a unitary laser beam 102 thatcomprises pre-pulses. The graph 600 is an actual example of the outputillustrated in FIG. 3. The unitary laser beam 102 is generated by asingle source but can be generated by multiple sources in other systems.In an embodiment, the pre-pulses have a wavelength of 10.26 microns. Asdepicted, the burst 602 has a duration of approximately 3.5 millisecondsand comprises a pre-determined number of laser pulses. According tovarious embodiments, a burst 602 may comprise different number of laserpulses based on the burst length. The laser pulses have a width(measured as a length of time) and an amplitude. As part of calculatingthe calibration coefficient, over a length of time shown in the graph600, the calibration module 904 can integrate the pulses of the burst602.

The calibration coefficients for the unitary laser beams 102 arecalculated in the same manner for both the unitary beam comprising mainpulses and the unitary beam comprising pre-pulses. First, thecalibration module 904 determines the energy of the burst of the laserpulses from the average power. The energy produced during the definedperiod of time over which the power was measured is determined:E _(total) =P _(measured) *T _(period)Where E_(total) is the energy of the laser beam 102 over the definedperiod of time, P_(measured) is the power measurement taken by the powermeter 202, T_(period) is the defined period of time of the power meter202 (e.g., one second). From E_(total), the amount of energy within aburst is calculated using the burst rep rate:

$E_{burst} = \frac{E_{total}}{f_{burst}}$Where E_(burst) is the energy of a burst, E_(total) is the energy of theunitary laser beam 102 over the defined period of time (e.g., onesecond), and f_(burst) is the burst rep rate.

To determine the calibration coefficient, K_(p), the following equationis used:K _(p) ∫Vdt=E _(burst)so that,

$K_{p} = \frac{E_{burst}}{\int{V{\mathbb{d}t}}}$Where K_(p) is the calibration coefficient, V is the voltage signalreceived from the PEM detector 208 such that the integral, ∫Vdt, is thearea under the curve of the voltage signal provided by the PEM detector208 over the length of time of the burst, and E_(burst) is the energy ofa burst determined from the average power data received from the powermeter 202. The calibration coefficient, K_(p), has units of Watts perVolt.

The SPEC module 906 is configured to calculate the energy of a singlepulse using the calibration coefficient calculated by the calibrationmodule 904. The SPEC module 906 receives voltage data from the PEMdetector 208 that comprises a temporal profile of a single pulse in aunitary laser beam (e.g., single main pulse 502 or single pre-pulse702).

FIG. 5 is a graph 500 output by the PEM detector 208 depicting atemporal profile of a single main pulse 502 in the unitary laser beam102. The single main pulse 502 may be a pulse within the burst 402 ormay be a pulse in a subsequent burst. The single main pulse 502 iscaptured by reducing the scope window 306 of the PEM detector 208. Themain pulse 502 has an amplitude and a width (measured as length oftime). As part of calculating an energy of the pulse 502, the SPECmodule 906 can integrate the main pulse 502.

FIG. 7 is a graph 700 output by a PEM detector depicting a temporalprofile of a single pre-pulse 702 in the unitary laser beam 102. Thesingle pre-pulse 702 is captured by reducing the length of time overwhich the PEM detector 208 measures the laser beam 102. The pre-pulse702 has an amplitude and a width (measured as a length of time). As partof calculating the energy of the pre-pulse 702, the SPEC module 906 canintegrate the pre-pulse 702.

Using the temporal profile of a single pulse, the energy of the singlepulse is calculated according to the formula:E _(pulse) =K _(p) ∫VdtWhere E_(pulse) is the energy of the pulse, K_(p) is the calibrationcoefficient for pulses of the wavelength of the pulse being measured,and V is the voltage signal received from the PEM detector 208 depictinga temporal profile of the pulse being measured such that the integral,∫Vdt, is the area under the curve of the voltage signal provided by thePEM detector 208 over the length of time of the pulse.

The optional recalibration module 908 is configured to determine whetherto recalibrate the PEM detector 208. The calibration coefficients maylose accuracy over time due to, for example, instrument drift, equipmentdeterioration, or degradation of the beam splitter from which the laserbeam is received. In a unitary laser beam, during a subsequent burst oflaser pulses (e.g., burst 402 or burst 602), the recalibration module908 is configured to compare the power meter 202 measurement to acalculated power of the laser beam 102 using data provided by the PEMdetector 208. As described herein, the comparison is made using a timeperiod of one second. As will be apparent to those skilled in the artbased on the following description, other time periods may be used, suchas a burst length 304 or a burst period 308, or several burst periods.

To calculate the power of the pulses of the laser beam 102 over a periodof one second, the energy of the pulses over a burst (e.g., burst 402 or602) is determined using the calibration coefficient:E _(burst) =K _(p) ∫VdtWhere K_(p) is the calibration coefficient, V is the voltage signalreceived from the PEM detector 208 such that the integral, ∫Vdt, is thearea under the curve of the voltage signal provided by the PEM detector208 over the length of time of the burst, and E_(burst) is thecalculated energy of a burst. The sum of laser pulse energy is used todetermine the total energy of the laser beam 102 over the time period:E _(total) =ΣE _(burst)Where E_(burst) is the energy of a burst, E_(total) is the calculatedenergy of the laser beam 102 over the defined period of time (e.g., onesecond), and ΣE_(burst) is the sum of laser pulse energy over thedefined period of time. To determine the power of the pulses, the totalenergy is divided by the time period (e.g., one second):

$P_{calculated} = \frac{E_{total}}{T_{period}}$Where E_(total) is the calculated energy of the laser beam 102 over thedefined period of time, P_(calculated) is a power value calculated fromthe voltage signal received from the PEM detector 208, and T_(period) isthe defined period of time (e.g., one second).

To determine whether to instruct the calibration module 904 torecalculate the calibration coefficient, the recalibration module 908,can calculate a difference between the calculated power and the measuredpower. The difference may be expressed as percentage. To determinewhether to recalibrate, the recalibration module 908 may compare thedifference to a threshold. In some instances, if the two power valuesare more than 15% off, the recalibration module 908 instructs thecalibration module 904 to recalculate the calibration coefficient. Basedon the comparison, the recalibration manager 908 can instruct thecalibration module 904 to update the calibration coefficient byrecalculating the calibration coefficient during a subsequent burst ofthe pulses of the laser beam 102.

When the energy monitor 200 or 902 is placed in the LPP EUV system 100so as to receive a combined laser beam 102, the system 900 of FIG. 9 isfurther configured to determine calibration coefficient used todetermine the energy of a pre-pulse in a combined laser beam 102. Theenergy monitor 902 is located in the LPP EUV system 100 to measure acombined laser beam 102.

To calculate the calibration coefficient of the pre-pulses in a combinedlaser beam 102, the calibration module 904 uses a different set ofcalculations than those used when calibrating for a unitary laser beam.These calculations use the calibration coefficient calculated for theunitary laser beam 102 of the main pulses to determine a portion of thepower measured by the power meter 202 attributable to the main pulses,then use the remaining portion of the power to determine the calibrationcoefficient for the pre-pulses in the combined laser beam.

FIG. 8 is an example temporal profile 800 of the voltage signal outputby a PEM detector 208 of a combined pulse comprising a pre-pulse and amain pulse separated by a length of time. The combined laser beam 102 isgenerated by combining the unitary laser beams 102 into a single laserbeam so that, within a burst of the combined laser beam 102, thepre-pulses of the burst 602 alternate with the main pulses of the burst402. As depicted in FIG. 8, a pre-pulse 802 precedes a main pulse 804 by15 microseconds. The laser pulses have a width (measured as a length oftime) and an amplitude. Over a length of time shown in the graph 800,the calibration module 904 and the SPEC module 906 can integrate thepre-pulse 802 separately from the main pulse 804. The integral is usedto determine the calibration coefficient for calculating the energy ofthe pre-pulse and to calculate the energy of subsequent pre-pulses in acombined laser beam.

The calibration module 904 calculates the power of the main pulses ofthe combined beam based on a portion of the voltage signal provided bythe PEM detector 208 indicating the temporal profile of the main pulsewithin the combined pulse. Using the temporal profile, the energy of themain pulse is calculated according to the formula:E _(main pulse) =K _(mp) ∫VdtWhere E_(main pulse) is the energy of the main pulse, K_(mp) is thecalibration coefficient for the main pulses calculated for the unitarylaser beam 102, and V is the voltage signal received from the PEMdetector 208 depicting a temporal profile of the main pulse beingmeasured such that the integral, ∫Vdt, is the area under the curve ofthe voltage signal provided by the PEM detector 208 over the length oftime of the main pulse.

The average power of the combined pulse is measured by the power meter202 over the defined period of time (e.g., one second). Based on theenergy of the main pulse, the power attributable to the main pulses overthe defined period of time is calculated as:

$P_{{main}\mspace{14mu}{pulse}} = \frac{\sum\; E_{{main}\mspace{14mu}{pulse}}}{T_{period}}$Where E_(main pulse) is the energy of the main pulse, P_(main pulse) isthe calculated power of the main pulses, T_(period) is the definedperiod of time used by the power meter 202, and ΣE_(main pulses) is thesum of the energy of the main pulses occurring over the defined periodof time used by the power meter 202. To determine the portion of thepower measured by the power meter 202 that is attributable to thepre-pulses, the calibration module 904 determines the difference:P _(pre-pulse) =P _(measured) −P _(main pulse)where P_(pre-pulse) is the portion of the power measured by the powermeter attributable to the pre-pulses of the combined pulse over thedefined period of time, P_(measured) is the power of the combined pulsemeasured by the power meter 202, and P_(main pulse) is the calculatedpower of the main pulses.

Using the power attributable to the pre-pulses, the energy attributableto the pre-pulses is determined by:

$E_{{pre} - {pulse}} = \frac{P_{{pre} - {pulse}}}{T_{period}}$Where E_(pre-pulse) is the total energy of the pre-pulse over thedefined period of time used by the power meter 202, P_(pre-pulse) is theportion of the power measured by the power meter attributable to thepre-pulses of the combined pulse over the defined period of time,T_(period) is the defined period of time of the power meter 202 (e.g.,one second).

To determine the calibration coefficient of the pre-pulses in thecombined laser beam, the following formula is used:

$K_{pp} = \frac{E_{{pre} - {pulse}}}{\int{V{\mathbb{d}t}}}$Where K_(pp) is the calibration coefficient of the pre-pulses in thecombined laser beam, V is the voltage signal received from the PEMdetector 208 such that the integral, ∫Vdt, is the area under the curveof the voltage signal provided by the PEM detector 208 over at least aportion of the defined period of time, and E_(pre-pulse) is the totalenergy of the pre-pulse over the defined period of time used by thepower meter 202. The calibration coefficient, K_(pp), has units of Wattsper Volt.

Once the calibration coefficient of the pre-pulses in the combined laserbeam is determined, the SPEC module 906 receives voltage data from thePEM detector 208 that comprises a temporal profile of a pair of apre-pulse and a main pulse in a combined laser beam. The SPEC module 906can then determine the energy of a subsequent pre-pulse using theformula:E _(pre-pulse) =K _(pp) ∫VdtWhere E_(pre-pulse) is the energy of the single pre-pulse, K_(pp) is thecalibration coefficient for pulses of the pre-pulse in the combinedlaser beam 102, and V is the voltage signal received from the PEMdetector 208 depicting a temporal profile of the pre-pulse beingmeasured such that the integral, ∫Vdt, is the area under the curve ofthe voltage signal provided by the PEM detector 208 over the length oftime of the pre-pulse.

The optional recalibration module 908 can further determine whether torecalibrate the PEM detector 208 using the power meter 202 for thepre-pulses in the combined laser beam 102, as described above. For thecombined laser beam, the recalibration module 908 determines the powerof the pulses by summing the energy of the pulses in the combined laserbeam over a defined period of time corresponding to the power meter 202.The recalibration module 908 then compares a power calculated from thesum to the power measured by the power meter 202, as described above.

FIG. 10 is a flowchart of an example method 1000 of calculating theenergy of a pulse, according to an example embodiment. The method 1000may be performed by the system 900.

In an operation 1002, a PEM detector is calibrated using a power meterfor a beam of a first laser. The first laser can produce main pulses orpre-pulses in a unitary laser beam, as described above. FIG. 11 is aflowchart of an example method 1100 of calibrating a PEM detector usinga power meter to determine the energy of pulses within a unitary laserbeam. The method 1100 may be performed as part of operation 1002 by, forexample, the energy monitor 200 or 902 and the calibration module 904 ofthe system 900.

In an operation 1102, a power measurement is received from the powermeter (e.g., power meter 202). The power measurement indicates theaverage power of the pulses of the unitary laser beam 102 over a periodof time.

In an operation 1104, a voltage signal over a length of time is receivedfrom a PEM detector (e.g., PEM detector 208). The voltage signal is atemporal profile of a burst of the pulses of the unitary laser beam 102.The length of time over which data is collected by the PEM detector 208is at least one burst within the period of time of the power meter 202.

In an operation 1106, the calibration coefficient of the laser beam 102is calculated. The calibration coefficient is calculated as described inconnection with the calibration module 904. The calibration coefficientcan be calculated by the calibration module 904.

If the energy monitor 200 is measuring a unitary laser beam, the method1000 proceeds to an operation 1006, skipping operation 1004. In theoperation 1006, the energy of a pulse is calculated. The energy of thepulse is calculated, for example, as described elsewhere herein inconnection with the SPEC module 906. In some instances, the SPEC module906 performs the operation 1006.

In an optional operation 1008, a determination is made as to whether torecalibrate the PEM detector by, for example, the recalibration module908. The determination is performed by comparing a power calculated fromthe voltage signal provided by the PEM detector to the power measured bythe power meter. If the determination is made to recalibrate, the method1000 returns to operation 1002, or, in some instances, operation 1004.If the determination is made to not recalibrate, the method 1000 returnsto operation 1006.

When the laser beam measured by the energy monitor 200 is a combinedlaser beam, the method 1000 proceeds from the operation 1002 to theoperation 1004. The PEM detector is calibrated for a combined beam todetermine an energy of a pre-pulse within the combined beam. A secondlaser can produce pre-pulses in bursts, as described above, which arecombined with main pulses in a combined laser beam 102. In the operation1004, the calibration coefficient for the pre-pulses in the combinedlaser beam 102 is determined. The calibration coefficient for thepre-pulses in the combined laser beam 102 is determined separately fromthat of the pre-pulses in the unitary laser beam 102 because the opticalcomponents of the LPP EUV system 100 affect the relationship between thetemporal profile of the pre-pulse and the power measured by the powermeter 202 after the unitary laser beams 102 are combined. Thecalibration coefficient of the pre-pulses is determined based on adifference between the power measured by the power meter and the powerattributable to the main pulses in the combined laser beam.

FIG. 12 is a flowchart of an example method 1200 of calibrating a PEMdetector (e.g., PEM detector 208) using a power meter (e.g., power meter202) on a combined laser beam having pre-pulses and main pulses. Themethod 1200 is an example method of performing the operation 1004 of themethod 1000 when the laser beam measured by the energy monitor 200 is acombined laser beam. The method 1200 may be performed by, for example,the calibration module 904 of the system 900.

In an operation 1202, the voltage data is received from the PEM detector208 in the energy monitor 200 or 902. The voltage data is a temporalprofile of the combined laser beam 102, as depicted in FIG. 8. Thelength of time over which data is collected by the PEM detector 208 isat least a portion of the period of time of the power meter 202.

In an operation 1204, the power attributable to the main pulses isdetermined. The power of the main pulses is determined as described inconnection with the calibration module 904 and the SPEC module 906.

In an operation 1206, the power data is received from the power meter202. The power data indicates the average power of the pulses of thecombined laser beam 102 over a period of time.

In an operation 1208, the power attributable to the pre-pulses withinthe combined laser beam 102 is determined. The power of the pre-pulsesis determined as described in connection with the calibration module 904and the SPEC module 906.

In an operation 1210, the calibration coefficient of the pre-pulseswithin the combined laser beam 102 is calculated. The calibrationcoefficient is calculated as described in connection with thecalibration module 904.

Proceeding to the operation 1006 when the laser beam measured by theenergy monitor 200 is a combined laser beam, the energy of therespective main pulses and the pre-pulses in the combined laser beam arecalculated as described with respect to operation 1006, above. Thecalibration coefficient of the operation 1002 calculated for the unitarybeam of main pulses is used to calculate the energy of a main pulse inthe combined laser beam. To calculate the energy of the pre-pulse in thecombined laser beam, the calibration coefficient of the operation 1004is used. When the laser beam measured by the energy monitor 200 is acombined laser beam, the method 1000 can then proceed to optionaloperation 1008 as described above.

The disclosed method and apparatus has been explained above withreference to several embodiments. Other embodiments will be apparent tothose skilled in the art in light of this disclosure. Certain aspects ofthe described method and apparatus may readily be implemented usingconfigurations other than those described in the embodiments above, orin conjunction with elements other than those described above. Forexample, different algorithms and/or logic circuits, perhaps morecomplex than those described herein, may be used, as well as possiblydifferent types of laser beam generation systems.

Further, it should also be appreciated that the described method andapparatus can be implemented in numerous ways, including as a process,an apparatus, or a system. The methods described herein may beimplemented by program instructions for instructing a processor toperform such methods, and such instructions recorded on a non-transitorycomputer readable storage medium such as a hard disk drive, floppy disk,optical disc such as a compact disc (CD) or digital versatile disc(DVD), flash memory, etc., or communicated over a computer networkwherein the program instructions are sent over optical or electroniccommunication links. It should be noted that the order of the steps ofthe methods described herein may be altered and still be within thescope of the disclosure.

It is to be understood that the examples given are for illustrativepurposes only and may be extended to other implementations andembodiments with different conventions and techniques. While a number ofembodiments are described, there is no intent to limit the disclosure tothe embodiment(s) disclosed herein. On the contrary, the intent is tocover all alternatives, modifications, and equivalents apparent to thosefamiliar with the art.

In the foregoing specification, the invention is described withreference to specific embodiments thereof, but those skilled in the artwill recognize that the invention is not limited thereto. Variousfeatures and aspects of the above-described invention may be usedindividually or jointly. Further, the invention can be utilized in anynumber of environments and applications beyond those described hereinwithout departing from the broader spirit and scope of thespecification. The specification and drawings are, accordingly, to beregarded as illustrative rather than restrictive. It will be recognizedthat the terms “comprising,” “including,” and “having,” as used herein,are specifically intended to be read as open-ended terms of art.

What is claimed is:
 1. A system comprising: an energy monitor within alaser source of a laser-produced plasma (LPP) extreme ultraviolet (EUV)system, the energy monitor configured to measure laser pulses having asame wavelength and occurring in a burst, the energy monitor comprising:a power meter configured to measure an average power of the laser pulsesover a defined period of time, and a photoelectromagentic (PEM) detectorconfigured to provide a first voltage signal indicative of a temporalprofile of the burst of the laser pulses over at least a portion of thedefined period of time; a calibration module configured to determine acalibration coefficient based on the average power and the first voltagesignal, the calibration coefficient being a ratio of an energy of theburst of the laser pulses determined from the average power and anintegral of the first voltage signal; and a single pulse energycalculation (SPEC) module configured to determine an energy of asubsequent pulse of the series of the laser pulses based on thecalibration coefficient and a pulse integral of a second voltage signalprovided by the PEM detector indicating a temporal profile of thesubsequent pulse.
 2. The system of claim 1, further comprising arecalibration module configured to calculate an energy of a second burstbased on a third voltage signal indicative of a second temporal profileof the second burst.
 3. The system of claim 2, wherein the recalibrationmodule is further configured to compare the energy of the second burstto a second average power sensed by the power meter and to instruct tothe calibration module to update the calibration coefficient based onthe comparison.
 4. The system of claim 3, wherein the recalibrationmodule is configured to instruct the calibration module to update thecalibration coefficient if the comparison exceeds a threshold.
 5. Thesystem of claim 1, wherein the laser pulses are main pulses.
 6. Thesystem of claim 1, wherein the laser pulses are pre-pulses.
 7. A methodcomprising: measuring laser pulses having a same wavelength andoccurring in a burst using an energy monitor within a laser source of alaser-produced plasma (LPP) extreme ultraviolet (EUV) system, themeasuring comprising: receiving, from a power meter, an average power ofthe laser pulses measured over a defined period of time, and receiving,from a photoelectromagnetic (PEM) detector, a first voltage signalindicative of a temporal profile of the laser pulses sensed during atleast a portion of the defined period of time; determining a calibrationcoefficient based on the average power and the first voltage signal, thecalibration coefficient being a ratio of an energy of the laser pulsesdetermined from the average power and an integral of the first voltagesignal; and determining an energy of a subsequent pulse of the series ofthe laser pulses based on the calibration coefficient and an integral ofa second voltage signal provided by the PEM detector indicating atemporal profile of the subsequent pulse.
 8. The method of claim 7,further comprising calculating an energy of a second burst based on athird voltage signal indicative of a temporal profile of the secondburst.
 9. The method of claim 8, further comprising comparing the energyof the second burst to a second average power sensed by the power meterand updating the calibration coefficient based on the comparison. 10.The method of claim 9, wherein updating the calibration coefficient isbased on the comparison exceeding a threshold.
 11. The method of claim7, wherein the laser pulses are main pulses.
 12. The method of claim 7,wherein the laser pulses are pre-pulses.
 13. A non-transitory computerreadable medium having instructions embodied thereon, the instructionsexecutable by one or more processors to perform operations comprising:measuring laser pulses having a same wavelength and occurring in a burstusing an energy monitor within a laser source of a laser-produced plasma(LPP) extreme ultraviolet (EUV) system, the measuring comprising:receiving, from a power meter, an average power of the laser pulsesmeasured over a defined period of time, and receiving, from aphotoelectromagnetic (PEM) detector, a first voltage signal indicativeof a temporal profile of the burst of the laser pulses sensed during atleast a portion of the defined period of time; and determining acalibration coefficient based on the average power and the first voltagesignal, the calibration coefficient being a ratio of an energy of theburst of the laser pulses determined from the average power and anintegral of the first voltage signal; and determining an energy of asubsequent pulse of the series of the laser pulses based on thecalibration coefficient and an integral of a second voltage signalprovided by the PEM detector indicating a temporal profile of thesubsequent pulse.